Saturday 15 June 2013 12.35 EDT
First published on Saturday 15 June 2013 12.35 EDT

The American secretary of state, John Kerry, warned on Saturday that what the US believes to be Syria's use of chemical weapons against rebels and the creeping role played by Hezbollah in the conflict "threatened to put a political settlement out of reach".

In a phone call with Iraq's foreign minister, Hoshyar Zebari, Kerry reaffirmed that the US was seeking a non-military solution to the crisis, but doubted President Bashar al-Assad's "commitment" to negotiations, according to a state department statement.

The news comes amid reports in the US media that the CIA is poised to begin arming rebels in Syria through secret bases in Turkey and Jordan. The use of clandestine hubs to ship weapons and ammunition to anti-Assad forces could begin within weeks, the Washington Post claimed, citing an unnamed US official.

The White House announced on Thursday that it believed there was concrete evidence of the use of sarin nerve gas by pro-government fighters against rebel groups. "The intelligence community estimates that 100 to 150 people have died from detected chemical weapons attacks in Syria to date; however, casualty date is likely incomplete," a White House statement read.

If true, it would confirm that Assad loyalists had crossed a "red line" set by President Barack Obama, triggering further intervention by the US and military support to rebels.

Damascus has responded by accusing Obama of lying over the use of chemical weapons as a pretext to America's involvement in the conflict. "The White House … relied on fabricated information in order to hold the Syrian government responsible for using weapons, despite a series of statement that conformed that terrorist groups in Syria have chemical weapons," a Syrian foreign ministry spokesman said.

Yuri Ushakov, foreign policy adviser to the Russian president, Vladimir Putin, cast doubt over the US assessment on the use of sarin gas. After being briefed by Washington, Ushakov noted that "what was presented to us by the Americans does not look convincing".

"It would be hard even to call them facts," he said.

Russia is likely to prove a stumbling block in efforts by the US to win broad international backing for an increase in support for rebel groups. Moscow has protected Assad from three UN security council resolutions aimed at forcing him to end the violence and has persistently opposed any foreign military intervention in the conflict.

Speaking on Saturday, the Russian foreign minister, Sergey Lavrov, further outlined his opposition to American intervention. Noting media reports that the US plan included the imposition of a no-fly zone inside Syria, Lavrov said such a move would violate international law.

The comment referred to an article in the Wall Street Journal that suggested Washington intended to provide air protection for rebels, enforced by US and allied planes to be launched from Jordan. The standoff between the US and Russia over Syria is likely to come to a head next week, when Obama is due to hold one-on-one talks with Putin on the sidelines of the G8 summit in Northern Ireland.